Purpose: To examine a method to visualize a 3‐dimensional (3D) rendered distal femur using 3D curvature analysis and to compare models of patellofemoral instability (PFI) with controls to study the reliability of the entry point to trochlear groove angle (EPTG) metric. Methods: The 3D models of patients with recurrent patellar instability, defined by at least 2 reported patellar dislocation events, and age‐ and sex‐matched controls were created from computed tomography scans. Curvature was calculated to highlight the proximal trochlear ridges and the trochlear groove by overlaying them on the 3D models. Anteroposterior views with and without curvature visualization were created and used for qualitative comparison and to measure the EPTG. The EPTG was measured by 2 raters with and without the aid of the curvature maps. Significant differences between patients with PFI and controls were compared with a Mann‐Whitney U test. Inter‐rater reliability was calculated using interclass correlation coefficients, classified according to literature and compared using a permutation test. Significance was assumed at .05. Results: Qualitive analysis between 30 PFI patient knees (age: 23.9 ± 8.4 years, female/male: 24/6) and 30 control knees (age: 21.8 ± 5.6 years, female/male: 22/8) showed that in general, patients with PFI have a lateralized medial ridge and trochlear groove, with the trochlear groove being shorter and shallower. Qualitatively, differences between patients with PFI and controls were significant for measurements both with and without the aid of the curvature maps. Inter‐rater reliability was significantly ( P = .0349) better when using the curvature visualization. Conclusions: Curvature‐based visualization aids overlain on a 3D model have the power to increase the information gained from 3D imaging and corresponding 3D models, amplifying their potential value in clinical decision‐making. Such visualizations facilitate both the identification of qualitative differences between patient and control morphology and improve the reliability of the EPTG trochlear dysplasia metric. Level of Evidence: Level III, retrospective cohort study.

Background: Tibial tuberosity–trochlear groove distance (TT-TG) is often used as a primary metric for surgical decision-making in the treatment of patellofemoral instability (PFI), particularly when considering tibial tubercle transfer. Although TT-TG has high interrater reliability, it is prone to measurement differences caused by the alignment of the patient's leg in a scanner gantry, potentially influencing surgical decision-making. Quantification of this error within the clinical literature remains limited. Purpose: To quantify and specify the error in TT-TG caused by leg-scanner alignment by using detailed topographical landmarks and 3-dimensional (3D) analysis of computed tomography scans of patients with PFI. Study Design: Controlled laboratory study. Methods: Three-dimensional models of knees with PFI were created from computed tomography scans and used to identify TT-TG landmarks. TT-TG was measured using the established 2-dimensional (2D) and 3D methods. A model to estimate the differences between these 2 methods was created, and the orientation of the patients’ legs in relation to scanner longitudinal axis was measured to validate this model via linear regression. Interrater reliability was calculated via intraclass correlation coefficients (ICC). Results: A total of 44 knees of patients with PFI were analyzed. Differences between the 2D and 3D methods ranged from -4.0 to 14.7 mm (mean ± SD, 2.7 ± 4.1 mm) with a root mean square difference of 4.8 mm. The TT-TG distance of the 2D method (19.8 ± 7.2 mm) was significantly ( P = .045) longer than that of the 3D method (17.1 ± 4.9 mm). The variance of the 2D method was significantly larger than that of the 3D method. In total, 13 (29.5%) of the knees had a difference of >5 mm between 2D and 3D TT-TG. The estimation model had an adjusted r ² value of 1.00 and a resulting root mean square difference of 0.21 mm. 3D TT-TGs interrater reliability was good to excellent (ICC, 0.94 [95 CI%, 0.81-0.98]). Conclusion: 3D TT-TG can be used to correct scanner-leg alignment errors, some of which are substantial when using only 2D TT-TG measurements. Clinical Relevance: The findings in this study suggest a need for caution and awareness of the potential effects of differences in alignment of the axes of the leg and scanner when using purely 2D TT-TG as a basis for surgical planning.

Background: The relationship between patellofemoral (PF) morphology and PF cartilage damage in the general population remains unclear. Purpose: This study aimed to determine whether 3-dimensional–based metrics of PF morphology are associated with progressive lateral PF cartilage damage. Study Design: Cross-sectional study; Level of evidence, 2. Methods: We analyzed nonweightbearing computed tomography scans of knees from a subset of participants enrolled in the community-based Multicenter Osteoarthritis Study. Baseline and 2-year magnetic resonance imaging scans of the knee were evaluated for progressive PF cartilage damage using the Magnetic Resonance Imaging Osteoarthritis Knee Score. Tibial tubercle–trochlear groove (TT-TG) distance, patellar tilt, external tibiofemoral rotation (eTFR), patellar height, entry point–trochlear groove angle, and entry point–transition point (EP-TP) angle were measured for each knee. To assess the association of each morphology measure with progressive cartilage damage, logistic regression models with generalized estimating equations were fit using continuous and natural cubic spline models. Results: We analyzed lateral PF cartilage damage in 389 knees (mean age, 53.79 ± 5.51 years; mean body mass index, 28.48 ± 5.13 kg/m ² ). TT-TG distance (β = 0.23; odds ratio, 1.26; P = .036), eTFR (β = 0.24; odds ratio, 1.27; P = .048), and EP-TP angle ( Z = 2.09; P = .036) all demonstrated significant positive associations with worsening lateral PF cartilage damage. Conclusion: The results demonstrated significant associations between 3-dimensional anatomic metrics and progressive lateral PF cartilage damage. Elevated TT-TG distance, eTFR, and EP-TP angle may be keys to understanding the mechanical cause of lateral PF osteoarthritis.

Tibial tuberosity to trochlear groove (TT-TG) distance serves as one of the main metrics for patellofemoral instability (PFI) surgical decision-making. The purpose of this study is to split TT-TG into translational (caused by bony morphology) and rotational (external tibiofemoral rotation) components, elucidate how those two components relate to each other, and determine how the components differ between recurrent PFI patients and controls. Computed tomography (CT) scans of PFI patients with at least two reported dislocation events, seen by our institution's orthopedic department, were retrospectively acquired. Control CT scans were acquired from the Multicenter Osteoarthritis Study (MOST). Three-dimensional (3D) landmarks were placed on the distal femora and the proximal tibias. TT-TG, its rotational and translational components, and tibiofemoral rotation were algorithmically calculated from these landmarks. The two cohorts' means were compared using Mann-Whitney U-tests. Pearson coefficients were used to evaluate the correlation between the TT-TG components. The reliability of the measurements was evaluated with intraclass correlation coefficients (ICCs). The minimal sample size for a power level of 0.80 was calculated with an a priori sample size calculation. A total of 26 PFI (sex parity; age: 24.6 ± 10.0 years) and 294 control knees (sex parity; age: 52.6 ± 7.0 years) were analyzed. Statistically significant differences for TT-TG (18.7 ± 4.8 vs. 12.0 ± 3.4 mm, p < 0.001), rotational (5.3 ± 2.5 vs. 1.0 ± 2.5 mm, p < 0.001) and translational (13.4 ± 3.7 vs. 11.0 ± 3.1 mm, p = 0.002) components of TT-TG, and tibiofemoral rotation (10.7 ± 4.7 vs. 1.9 ± 4.7°, p < 0.001) were found. No significant correlation between the components of TT-TG was found (p = 0.14, r² = 0.29). Predictive ICCs for the four measurements ranged from 0.82 to 0.99. TT-TG can be split into (1) a translational component, primarily dependent on bony morphology, and (2) a rotational component, caused by external tibiofemoral rotation, both of which can lead to an elevated TT-TG measurement independently of each other. The rotational component is the primary factor for differences observed between PFI patients and controls but might vary between consequential patient scans. Our findings emphasize the importance of personalized treatment strategies tailored to individual patient profiles in treating patellar instability and will aid in more accurately targeted selection of surgical methods addressing either or both translational or rotational components of TT-TG. III Case-Control study.

Background: Trochlear dysplasia is important in recurrent patellofemoral instability (PFI); reliable metrics are essential to understand this condition and help define optimal treatment. One such measurement is the entry point to transition point (EP-TP) angle, which describes 2 components of patellar tracking from full knee extension to flexion: (1) mediodistal movement and (2) pure distal movement. Currently, measurement of this metric relies on 3-dimensional (3D) models. Purpose: To further develop the EP-TP angle metric to allow its digital measurement, to establish cohort and interrater reliability values, and to evaluate its appropriateness for clinical use. Study Design: Cohort study (diagnosis); Level of evidence, 3. Methods: A data set composed of 30 recurrent PFI patient knees and 30 control knees was acquired. Each knee came with anterior views of the distal femur and curvature maps, which highlighted relevant structures such as the trochlear groove and the medial and proximal ridge. Three raters measured EP-TP angles utilizing a digital tool with and without the curvature maps. Significance of differences in means, presence of a TP, and interrater reliability with intraclass correlation coefficients were tested and evaluated according to an established guideline (0.5-0.75, moderate; >0.75-0.9, good; >0.9 excellent reliability). A significance level of .05 corrected with the Bonferroni method was assumed. Results: PFI knees had a significantly higher EP-TP angle than the control cohort (48.9° ± 10.8° vs 11.8° ± 14.6°; P < .001). Patients had a greater TP prevalence than controls (96.7% vs 10.0%). Interrater reliability for the EP-TP angle with curvature maps was significantly better than without (0.84 [95% CI, 0.76-0.89] vs 0.67 [95% CI, 0.55-0.78]; P < .001). Conclusion: The EP-TP angle is significantly different between PFI patients and controls. This study presents a digital approach for measuring the EP-TP angle, in contrast with the initially described method, which required 3D prints. This digital approach, when aided with curvature mapping, yields better reliability.